A prior art hydropneumatic sphere is shown in axial cross-section in FIG. 1A.
This prior art sphere comprises a rigid outer jacket 1, for example one made of metal, having an upper portion 1a and a lower portion 1b which are welded together or formed as one piece, and a pliable deformable membrane 2 fastened inside said jacket in the area of junction 1c between these upper and lower portions, so as to delimit an upper chamber 3, which encloses a gas, for example, and a lower chamber 4 designed to contain a fluid, for example a liquid.
The upper chamber 3 is intended to contain a pressurized gas injected through an inflation screw 5 set in the top of the jacket 1, which is screwed into a position-retention element 6 in the form of a spherical cap housed inside the upper portion 1a of the jacket 1.
The lower chamber 4 connects, for example, with a hydraulic circuit (not shown) by means of a connection end piece 7.
The membrane can be shifted inside the jacket 1 between an extreme lowered position, in which it is in contact with the connection end piece 7, and an extreme raised position, in which it comes into contact with the inflation screw 5.
In the extreme lowered position, the membrane 2 is generally hemispherical in shape, and the equatorial peripheral edge 2a incorporates a radially- and inwardly-projecting bulge which grips a circular collar 8 in the aforementioned area of junction 1c, in order to secure attachment of the membrane thereto.
The membrane 2 is attached to the collar 8 in such a way that, in the lowered position, it extends locally from the area of junction 1c in a direction D parallel to the axis A of the sphere.
The membrane 2 is thicker in its central area 2b, in which is embedded an attachment stub 9a made integral with a disk-shaped plate 9, which is attached beneath this stiffened central area and designed to come into contact with the connection end piece 7 in the extreme lowered position.
This type of sphere is already in use as a replacement for the spring of a MacPherson suspension, the fluid used to actuate the suspension thrustor establishing a fluid connection with the lower portion of the sphere, so that, when the car body is depressed, the fluid is pushed back into the lower chamber 4, thereby compressing the gas enclosed in the upper chamber 3 on the other side of the membrane 2, the expansion of the gas compressed in this way ensuring the return of the fluid into the hydraulic circuit of the suspension when the vehicle wheels undergo spring movement.
In this conventional type of sphere, the membrane is subjected to strong mechanical stresses during its motion between the lowered and raised positions and undergoes repeated tractive stresses, with the result that the membrane can gradually become cracked and lose fluid-tightness, and that it becomes necessary to reinflate periodically the upper chamber of the sphere.
Furthermore, during the shift from the lowered to the raised position, the curvature of the membrane varies greatly, thus causing a so-called "blistering" phenomenon.
FIG. 1A shows, moreover, that in the extreme raised position of the membrane 2, a sizeable non-functional volume persists in the upper chamber.